Across Latin America, virtually no empirical data exist on the subject of sex trafficking, defined by the United Nations as forced, coerced, fraudulent or deceitful entry in to sex work, entry by abduction, or entry into such work under age 18. Recent media reports support that trafficking of women and girls into sex work, and drug cartel involvement in this form of sexual exploitation, is on the rise in the U.S. Mexico border region, constituting a major human rights crisis. Further, previous research by the PI in South and Southeast Asia has established that entry into sex work via trafficking confers elevated HIV/STI risk. Although the nature and context of sex trafficking in the US-Mexico border region likely differ significantly from that of Asia (e.g., the likely role of illicit drugs), research of our binational research team has found that factors often associated with trafficking among FSWs (e.g., young age, violence) relate to increased HIV risk, illicit drug use and forced injection drug use in Tijuana and Ciudad Juarez, suggesting that sex trafficking may be promoting vulnerability to HIV in this region. To both prevent sex trafficking and reduce risk for HIV among women and girls who have been trafficked, data is required a) on how sex trafficking is occurring and what structures facilitate this crime, and b) on whether and how victims, once trafficked, are made vulnerable to HIV. Despite the clear and urgent need in the region, no study of these issues has been conducted to date. The proposed study aims to describe the nature of sex trafficking in two major border cities (Tijuana and Ciudad Juarez) including major structural factors (e.g., those related to drug cartel activities, police violence, northward migration, access to health services) that may affect both sex trafficking and HIV risk among trafficking victims. To accomplish this, we propose to conduct a mixed-methods study of 800 FSWs from Tijuana and Ciudad Juarez in consultation with multiple national and state-level systems key informants (SKI) to inform the content, ensure the relevance, and maximize the acceptability and dissemination of our work. As an estimated 70% of FSWs in these cities originate from other Mexican states or Central and South America, these locations offer an unparalleled opportunity within Latin America to study these concerns, as well as to inform the development of much- needed interventions. In-depth interviews will be conducted with 30 FSWs (15 per city) screening positive for sex trafficking (minor or forced/coerced sex work entry). Data will be collected from 800 FSWs (400 per city) identified using venue-based sampling, via self-report survey and biologic HIV/STI testing. Based on interview, survey and laboratory data and SKI consultation, targets will be identified and prioritized for region-specific, structural interventins (i.e., those focused structural factors found to be associated with sex trafficking and HIV risk among trafficked FSWs) to prevent sex trafficking and reduce coincident HIV risk along the US.- Mexico border. Development and piloting of indicated structural interventions will be proposed within a subsequent R34 application.